Cell motility influences many biological processes. During development, cells move in an organized manner to give rise to specific organs, and disruption of this process can lead to serious abnormalities. Directed cell movements are also associated with angiogenesis, neuronal re-arrangements, wound healing, and the movement of macrophages to sites of infection. One of the most lethal manifestations of uncontrolled cell motility is metastasis. In muscle, myosin is the protein which converts the chemical energy of ATP into the mechanical energy of movement. Myosin is also present in non-muscle cells and is believed to be responsible for most types of cell locomotion. In order to understand how cell motility is regulated, therefore, it is essential to study regulation of the myosin molecule. Dictyostelium is a good system for investigating this problem because the amoebae can be grown easily, are amenable to genetic analysis, and exhibit chemotaxis. The heavy chain of the amoebae myosin is phosphorylated in vivo, and this modification inhibits self-assembly and ATPase activity. In contrast, phosphorylation of the light chain enhances ATPase activity. The goals of this research are to understand how the cell regulates the level of phosphorylation, and how this modification exerts its effect on myosin function. To qualtitate changes in phosphorylation during chemotaxis, cells are grown in radioactive phosphate and myosin is rapidly purified by immunoprecipitation. Because the level of phosphorylation is ultimately controlled by specific kinases and phosphorylation is ultimately controlled by specific kinases and phosphatases it is important to purify these enzymes. Studies on the regulation of these enzymes should help elucidate the chain of events which links an extracellular signal to changes in myosin phosphorylation. These purified enzymes also provide the tools for altering levels of phosphorylation in vitro. Current in vitro studies concern the effects of phosphorylation on ATPase activity and assembly. In addition studies are being carried out to determine if the supramolecular organization of molecules within the thick filament can control ATPase activity.